feat: add wiki build kind with file-structure-derived navigation

- add build_wiki_nav deriving MkDocs nav from docs/wiki file structure
  (index.md -> Home, numeric prefixes stripped and title-cased, nested
  dirs become groups, natural ordering)
- add --wiki / --wiki-dir to build; wiki-only builds need no --module
- merge wiki nav before generated lib/api nav; wiki Home replaces the
  nav spec Home entry
- add mkdocs.wiki.yml template fragment and nav/cli tests
- dogfood doc-forge's own docs/wiki and regenerate site output

docs/wiki/02_architecture.md

@@ -0,0 +1,34 @@
+# Architecture
+
+`docforge` is split into four horizontal layers. Everything flows top to
+bottom through the CLI.
+
+## Loaders → Models
+
+The `docforge/loaders` package wraps `griffe` to extract modules, functions,
+classes, and Google-style docstring sections. Loaded data is normalized into
+the object model under `docforge/models` (`Module`, `Object`, `Project`,
+`Field`).
+
+## Navigation
+
+`docforge/nav` parses `docforge.nav.yml` specs (`NavSpec`, `Resolver`, and the
+MkDocs nav emitter) and, since the wiki kind, derives wiki navigation from the
+file structure via `build_wiki_nav`.
+
+## Renderers
+
+`docforge/renderers` turn model data into artifacts:
+
+- `MkDocsRenderer` → `docs/lib/**` reference markdown
+- `MCPRenderer` → `docs/mcp/**` structured documentation
+
+## CLI
+
+`docforge/cli` wires it all together:
+
+- `commands.py` — the `build` command and its `--mkdocs` / `--api` / `--wiki`
+  / `--mcp` modes
+- `mkdocs_utils.py` — config generation (`mkdocs.yml`) including merged
+  wiki + lib + api navigation
+- `api_utils.py` — OpenAPI loading and API docs generation